The present invention relates to a two-dimensional image detector that is suitably used for detecting a two-dimensional image formed with light (radiation) such as X-rays, visible light, or infrared rays, and a method for fabricating the same.
Conventionally well known as a detector for detecting a two-dimensional image formed with radiation is a two-dimensional image detector in which a plurality of semiconductor sensors that detect X-rays projected thereto and generate electric charges (electron-hole) are planar arranged, each semiconductor sensor being equipped with an electric switch. The two-dimensional image detector is arranged so as to detect a two-dimensional image by turning on the electric switches line by line successively to read electric charges generated by the semiconductor sensors in each line.
A principle and concrete structure of the foregoing two-dimensional image detector is taught by, for instance, the following documents: D. L. Lee et al., xe2x80x9cA New Digital Detector for Projection Radiographyxe2x80x9d, SPICE, 2432, pp.237-249, 1995xe2x80x9d; L. S. Jerkin et al., xe2x80x9cApplication of a-Si Active Matrix Technology in a X-ray Detector Panelxe2x80x9d, SID 97 DIGEST, pp.91-94, 1997; and the Japanese Publication for Laid-Open Patent Publication No. 342098/1994 (Tokukaihei 6-342098 [Date of Publication: Dec. 13, 1994]). The foregoing two-dimensional image detector is formed in a manner such that a photoconductive layer that absorbs X-rays to generate electric charges is formed on an active matrix substrate that is a substrate provided with a plurality of electrode wires arranged in XY matrix, switching elements and pixel electrodes provided at intersections of the foregoing electrode wires, and further, bias electrodes are provided on the foregoing photoconductive layer.
Then, for example, the foregoing document discloses an arrangement in which a-Se (amorphous selenium) that has good sensitivity with respect to X-rays and is easily formed on a large-area substrate is used as a material for forming the photoconductive layer. a-Se exhibits a high X-ray absorption factor and a high ratio of conversion of X-rays to electric charges (hereinafter referred to as ray-charge conversion ratio), and also, it is directly formed on an active-matrix substrate at a relatively low temperature by vapor deposition.
To improve the S/N ratio of the two-dimensional image detector, the quantity of electric charges generated by X-ray absorption should be increased. To increase the quantity of electric charges, it is necessary to form the photoconductive layer to a thickness of about 500 xcexcm to 1500 xcexcm. Formation of such a thick photoconductive layer by vapor deposition, however, takes long time, and further, management of the process is complex. This causes the productivity to become extremely low, thereby causing manufacturing costs of the two-dimensional image detector. Moreover, in the method in which a photoconductive layer is directly provided on the active matrix substrate, the active matrix substrate is heated upon formation of the photoconductive layer. Therefore, the heat resistance (heat resistance temperature) of the active matrix substrate has to be taken into consideration. Therefore, by the foregoing method, it is impossible to use a material that has to be laminated (for instance, CdTe, CdZnTe) as a material forming the photoconductive layer.
Then, as a method to solve the foregoing problem, the Japanese Publication for Laid-Open Patent Application No. 211832/1999 (Tokukaihei 11-211832 [Date of Publication: Aug. 6, 1999]) teaches a method for forming a photoconductive layer in a coating process, rather than vapor deposition. More specifically, in the foregoing method, a particle-dispersed material that is formed by dispersing a particle-form photoconductor in a binder such as resin with insularity is coated to a thickness of about 700 xcexcm to 3000 xcexcm over the active matrix substrate, to form a photoconductive layer. By this method, any one of various materials that exhibit high X-ray absorption factors and high ray-charge conversion ratios (photoconductive materials), apart from the aforementioned a-Se, can be used as the material composing the photoconductor. Therefore, by combining such a material and a binder such as resin, a photoconductive layer can be formed. Incidentally, since the foregoing method allows a photoconductive layer to be formed in a short time, it is considered that a high productivity can be achieved, while the manufacturing cost of the two-dimensional image detector can be reduced.
By the method taught by Tokukaihei 11-211832 mentioned above, a photoconductive layer is made of a particle-dispersed material that is formed by dispersing a particle-form photoconductor in a binder with insularity such as resin. In the case where the photoconductor particles are uniformly dispersed in an insulating binder, the photoconductor particles become hardly in contact with each other, and electric charges generated at each photoconductor particle are not smoothly conducted through the photoconductive layer. Therefore, there arises a problem that electric charges generated at the photoconductive layer cannot be conducted effectively to the active matrix substrate.
On the other hand, in view of an arrangement of a copying-machine-use photosensitive drum, the Japanese Publication for Laid-Open Patent Application No. 11-237478 (Tokukaihei 11-237478 [Date of Publication: Aug. 31, 1999]) discloses a method in which two layers of a charge transporting layer and a photoconductive layer are laminated in the stated order on an active matrix substrate so as to effectively conduct electric charges generated in the photoconductive layer to the active matrix substrate, that is, a method for improving charge collecting efficiency. The foregoing photoconductive layer is made of a particle-dispersed substance that is formed by dispersing a particle-form photoconductor in a binder with insularity such as resin. However, to produce a two-dimensional image detector that is to detect X-rays, it is necessary to form the photoconductive layer to a thickness of about 700 xcexcm to 3000 xcexcm, to improve the X-ray absorption factor. On the contrary, about 0.1 xcexcm to 1 xcexcm is sufficient as a thickness of the charge production layer (equivalent to the photoconductive layer) of the copying-machine-use photosensitive drum.
In the case where the photoconductive layer with the foregoing thickness is formed, however, the electric charges generated by the photoconductor particles tend to be prevented by the insulating binder from being conducted. In other words, electric charges generated by the photoconductor particles cannot be conducted smoothly through the photoconductive layer. Therefore, there arises a problem such that the electric charges generated in the photoconductive layer cannot be efficiently conducted to the active matrix substrate through the charge transporting layer.
In other words, in the above-described two-dimensional image detector equipped with the photoconductive layer, electric charges generated in the photoconductive layer cannot be efficiently conducted to the active matrix substrate. Thus, there is a problem such that it is difficult to provide a two-dimensional image detector that is superior in sensitivity with respect to X-rays, visible light, infrared, and other light (radiation).
The present invention was made in light of the foregoing problems of the prior art, and an object of the present invention is to provide a two-dimensional image detector and a producing method of the same, the two-dimensional image detector being arranged so that charges generated by each particle of a photoconductor are smoothly transmitted through a photoconductive layer even in the case where the photoconductive layer is formed with a dispersion in which the particulate photoconductor is dispersed in a binder such as resin with insularity, thereby resulting in efficient transmission of charges generated in the photoconductive layer to the active matrix substrate, that is, a two-dimensional image detector superior in sensitivity to light (radiation) such as X-rays, visible light, or infrared rays.
A first object of the present invention is to provide a two-dimensional image detector in which charges generated at each photoconductor particle are smoothly transmitted through the photoconductive layer, thereby realizing efficient transmission of charges generated in the photoconductive layer to the active matrix substrate.
To achieve the foregoing first object, a two-dimensional image detector in accordance with the present invention includes at least an active matrix substrate having a plurality of pixel electrodes, and a photoconductive layer laminated on the pixel electrodes, so that the photoconductive layer is made of a mixture of a particulate photoconductor and a binder containing a resin that renders volumetric shrinkage upon reaction.
According to the foregoing arrangement, the photoconductive layer is made of a mixture of a particulate photoconductor and a binder containing a resin that renders volumetric shrinkage upon reaction. Therefore, after the mixture is laminated on the active matrix substrate by coating, heat transfer (a photoconductive layer is formed on another substrate beforehand then transferred to the active matrix substrate) or the like, reaction (polymerization, cross-linking, or decomposition) of the resin can be promoted either by subjecting the mixture to heat treatment, irradiating the same with light (application of thermal energy or optical energy), or by allowing the same to stand in-atmosphere. Therefore, a photoconductive layer can be easily formed on the active matrix substrate. Since the resin renders volumetric shrinkage depending on the material of the binder upon the foregoing processing operation, particles of the photoconductor tend to approach each other, contacting and flocculating, even in the case where the particulate photoconductor is dispersed uniformly in the binder. This results in smooth transmission of charges generated at each particle of the photoconductor through the photoconductive layer, thereby realizing efficient transmission of charges generated in the photoconductive layer to the active matrix substrate. This enables provision of a two-dimensional image detector superior in the sensitivity to light (radiation) such as X-rays, visible light, infrared rays, etc.
Furthermore, the photoconductive layer 2 can be formed on the active matrix substrate 1 by a method other than vapor deposition that includes an evaporating process, that is, more concretely, by coating or heat transfer. Therefore, materials, forming methods, conditions for formation, etc. can be appropriately selected from a wider range (the range of selection is widened), as compared with the case where the photoconductive layer 2 is formed by vapor deposition on the active matrix substrate 1. In other words, since the photoconductive layer 2 can be formed without use of vapor deposition, the materials forming the photoconductive layer 2, the method and conditions of formation of the photoconductive layer 2, etc. can be appropriately selected from wider ranges of materials, method, and conditions, without the need to take the heat resistance of active matrix substrate, for instance. Therefore, various materials (photoconductive materials) that are not suitable for application by vapor deposition can be used as materials of the photoconductive layer 2, and the productivity of the photoconductive layer 2 can be improved. Furthermore, it is superior in uniformity of the film thickness and composition, and moreover, it enables easy formation of the photoconductive layer 2 to a thickness (film thickness) of about 700 xcexcm to 3000 xcexcm.
To achieve the aforementioned first object, a two-dimensional image detector in accordance with the present invention includes at least an active matrix substrate having a plurality of pixel electrodes, and a photoconductive layer laminated on the pixel electrodes, wherein the photoconductive layer is made of a mixture of a particulate photoconductor and a binder containing a charge transport medium.
According to the foregoing arrangement, the photoconductive layer is composed of a particulate photoconductor and a binder containing charge transport medium. Therefore, by applying a method such as coating, heat transfer (a photoconductive layer is formed on another substrate beforehand then transferred by heat to the active matrix substrate) or the like, a photoconductive layer can be formed on the active matrix substrate easier. Since the binder contains the charge transport medium, charges generated at each photoconductive particle are smoothly transmitted through the photoconductive layer even in the case where the particulate photoconductor is uniformly dispersed in the binder. Therefore, the charges generated in the photoconductive layer are efficiently transmitted to the active matrix substrate. This enables to provide a two-dimensional image detector superior in sensitivity to light (radiation) such as X-rays, visible light, infrared rays, etc.
The second object of the present invention is to provide a method for fabricating a two-dimensional image detector that provides smooth transmission of charges generated at each particle of the photoconductor through the photoconductive layer, thereby realizing efficient transmission of charges generated in the photoconductive layer to the active matrix substrate.
To achieve the foregoing second object, a method for fabricating a two-dimensional image detector of the present invention includes the step of laminating a photoconductive layer that is made of a mixture of a particulate photoconductor and a binder containing a resin that renders volumetric shrinkage upon reaction, on an active matrix substrate having a plurality of pixel electrodes.
To achieve the foregoing second object, another method for fabricating a two-dimensional image detector of the present invention includes the step of laminating a photoconductive layer that is made of a mixture of a particulate photoconductor and a binder containing a charge transfer medium, on an active matrix substrate having a plurality of pixel electrodes.
The foregoing arrangement enables to provide a method for fabricating a two-dimensional image detector that renders the aforementioned various effects.
For a fuller understanding of the nature and advantages of the invention, reference should be made to the ensuing detailed description taken in conjunction with the accompanying drawings.